1. Field of the Invention
This invention relates to a semiconductor laser device having a buried heterostructure.
2. Description of the Prior Art
Buried type semiconductor laser devices, in which an active layer for laser oscillation is surrounded by semiconductor layers having a refractive index smaller than that of the active layer and an energy gap larger than that of the active layer, are advantageous in that laser oscillation can be attained in a stable transverse mode at a low threshold current level, and accordingly they have been used as light source for optical communication systems and/or optical measuring systems. For these reasons, they are industrially important devices. However, with such buried type semiconductor laser devices, ineffective current not passing through the active layer greatly increases with an increase in current injected into the devices, which causes limitations to the maximum value of the output power of the devices. Moreover, the ineffective current increases with a rise in temperature, which causes difficulties in practical application of these buried type semiconductor laser devices, especially InGaAsP/InP semiconductor laser devices.
The reasons why the above-mentioned ineffective current arises seem to be as follows: Buried type semiconductor laser devices are, for example, produced in such a manner as shown in FIGS. 2 and 3. The laser device shown in FIG. 2, is produced as follows: On an n-InP substrate 1, an n-InP buffer layer 2, a non-doped InGaAsP active layer 3, and a p-InP cladding layer 4 are successively grown by an epitaxial growth technique. The resulting multi-layered epitaxial growth crystal is subjected to a chemical etching treatment to form a mesa. Then, on both sides of the mesa, a p-InP burying layer 5 and an n-InP burying layer 6 are grown. The laser device shown in FIG. 3 is produced as follows: On an n-InP substrate 1, a p-InP burying layer 5 and an n-InP burying layer 6 are successively grown by an epitaxial growth technique. The resulting epitaxial growth crystal is subjected to a chemical etching treatment to form a groove. Then, an n-InP buffer layer 2, an InGaAsP active layer 3, and a p-InP cladding layer 4 are successively grown in the groove.
The device produced according to the production mode shown in each of FIGS. 2 and 3 attains laser oscillation depending upon the injected current 7 passing through the active layer 3. Since the p-n junction at the interface between the burying layers 5 and 6 positioned at the sides of the active layer 3 is reversely biased, little current passes through the burying layers 5 and 6 when the injected current 7 is small. However, a considerable amount of current passes through the burying layers 5 and 6 positioned at the sides of the active layer 3 as the injected current 7 increases. This is because a thyristor composed of the cladding layer 4, the burying layer 5 and the buffer layer 2 (or the substrate 1) is made conductive by a gate current 7b which flows from the cladding layer 4 to the burying layer 5 (Higuchi et al: Laser Kenkyu Vol. 13, p. 156, 1985). If the active layer 3 is formed at the interface between the lower burying layer 5 and the upper burying layer 6, the injected current (i.e., the gate current) 7b will be reduced. However, such a precise control of the thickness of layers cannot be made using liquid phase epitaxy and chemical etching techniques at the present. Thus, the ineffective current mentioned above cannot be prevented.